Samuelon Stack Overflow • Recommends
On “How do I edit an incorrect commit message with TortoiseGit?”
For the case of when the commit is in the middle, I highly recommend not following the instructions provided by linquize, see the comments below his answer for the reason.
You can use the git command line with TortoiseGit and it won't cause any problems: Can I use command-line Git tools and TortoiseGit simultaneously?.
This youtube video explains it really well: link to this video. Basically use git rebase -i and then simply "reword" the commit message.
Update: I believe you can retrieve the lost commits from the hard reset suggested by linquize, see Wayne's answer here: How can I reorder/combine commits using Git rebase?.